It is desirable to measure circuit structures and other types of structures, e.g., resist structures, during the production of integrated circuits. Optical metrology tools are particularly well suited for measuring microelectronic structures because they are nondestructive, accurate, repeatable, fast, and inexpensive. Often different metrology tools are required to measure different structures or parameters on a wafer. For example, certain structures on a wafer act as diffraction gratings, which conventionally require a different metrology tool, e.g. critical dimension-scanning electron microscopy (CD-SEM), than is used to measure planar thin films.
One tool that is sometimes used to measure diffracting structures is a scatterometer. Scatterometry is an angle-resolved measurement and characterization of light scattered from a structure. Scatterometry is discussed in detail International Publication No. WO 99/45340, dated Sep. 10, 1999, which is incorporated herein by reference.
International Publication No. WO 99/45340 discloses the use of a spectroscopic ellipsometer to measure the diffracting structure. The sampling beam is incident on the sample at an oblique angle. The incident light of the spectroscopic ellipsometer is polarized to provide a beam in the TE mode (S-polarized) when the incidence plane of the beam is perpendicular to the grating of the diffracting structure or to provide a beam in the TM mode (P-polarized) when the incidence plane of the beam is parallel to the grating. Aligning the incident radiation with the grating of the diffracting structure unfortunately is difficult, particularly where the wafer stage is an r-θ stage. With an r-θ stage, the entire metrology apparatus must be rotated to properly align the incident radiation with the grating. International Publication No WO 99/45340 discloses a dedicated scatterometer instrument that uses a spectroscopic ellipsometer with non-normal incident light and that is used in a scatterometer mode.
In addition, International Publication No WO 99/45340 teaches that a reference database is generated using optical modeling. The reference database is simplified by measuring the film thickness and optical indices of film underlying the diffracting structure. Thus, prior to ellipsometrically measuring the diffraction grating, a measurement of the underlying film is performed. A broadband ellipsometric measurement is then made at a single polarization orientation, and the reference database is consulted to determine the structure of the diffraction grating. As can be seen, even though the size of the database is reduced by measuring the film thickness and optical indices of the underlying film, this process still requires the generation of a relatively large database. Further, the sample or metrology device must be moved and refocused to measure the underlying film, i.e., without the diffracting structure, and the diffracting structure itself, which is time intensive.
U.S. Pat. No. 5,963,329 by Conrad et al., issued Oct. 5, 1999 (the '329 patent), discusses a method of determining the line profile of a diffracting structure. The '329 patent discloses constructing a model of the diffraction structure and using rigorous coupled wave (RCW) analysis as described by Morham in the J.Opt.Soc.Am., Vol. 12, No. 5, May 1995, to calculate the electric and magnetic fields and light intensity reflected by the modeled diffracting structure. Unfortunately, RCW requires a large number of floating point operations. The RCW process uses both the negative and positive diffracted orders of the light in calculations. As a result, if, for example, 20 orders are used, 41 orders are actually calculated (Zeroth, order+20 negative orders+20 positive orders). Consequently, RCW requires the calculation and manipulation of 41 by 41 square matrices in the above example. The time required to perform the full RCW calculation is dominated by a single matrix eigenvalue calculation for each layer in the model of the diffracting structure at each wavelength, and numerous matrix multiplications. Both of these operations require at least N3 floating point operations. Accordingly, the process used in the '329 patent is time consuming and requires a large memory footprint.
Thus, what is needed is an optical metrology process to quickly and accurately measure diffraction gratings.